1. Introduction

     The chemical composition of unevolved metal-poor stars provides important information about the nucleosynthesis processes during the early galactic evolution. As these low mass stars do not dissipate orbital energy the knowledge of their kinematics provides new "fossil" information with which to explore various formation scenarios (Johnston et al. 1996).

2. The Galactic Mass Model

     We have adopted a slightly modified Ostriker and Caldwell (1979) model, whose components are:

• a central mass,
• a flattened disk whose surfaces of equal density are oblate spheroids,
• a spherical bulge-halo,
• a spherical dark halo.

2.1. Thr Density Laws

• Flattened Disk
  We adopt the following density profile _d for the disk component
  

  (1)

  where .
  R and z are the usual cylindrical coordinates, a and c being respectively the scale length and height of the disk.
  
  
• Spherical bulge-halo
  The bulge-halo density profile, _bh, is given by
  

  (2)

  where , n is the core radius and ₀ the central surface density of the bulge-halo.
  
  
• Spherical Dark Halo
  Finally, the density profile of the dark halo is
  

  (3)

  where _dh,0 is the central value and r_dh the core radius of the dark halo.

3. The Data

     The data sample was built from 3 different studies giving high quality abundance ratios of nearby unevolved metal-poors stars. These stars have a metallicity around [Fe/H] ~ -1, which corresponds approximately to the transition between the halo and the disk.
The 21 stars taken from Jehin et al. (1999) have high precision abundance ratios of 16 elements. Nissen and Schuster (1997) derived accurate abundances for 11 elements and have computed orbits with the model of Allen and Santillàn (1991) for 25 disk and halo stars. The 40 most metal-poor ([Fe/H] < -0.6) F and G disk stars come from the large study of Edvadsson et al. (1993), where they have obtained accurate abundances of a douzen elements in a differential analysis with respect to the sun, and the orbital parameters using a three-component Myamoto potential.

     The new parallaxes and proper motions are available in the HIPPARCOS Catalogue (ESA, 1997). The is better than 15 % for all these stars (except for 5 stars from Nissen and Schuster, 1997) with a mean error of 5.5 % for all the sample. Proper motions errors only reach 2 %. The radial velocities were selected from several sources: Barbier-Brossat et al. (1994), Evans (1967) and Wilson (1953). They are known to about 2 km/s for most of the stars.
We applied a systematic correction to the abundance ratios, based on stars which are common with the analysis of Nissen and Schuster (1997).

Acknowledgements

     We are grateful to Dominique Naef to have made available, before publication, the CORAVEL radial velocities of our stars. This work has been supported by contracts ARC 94/99-178 "Action de Recherche Concertée de la Communauté Française de Belgique" and by the Pôle d'Attraction Interuniversitaire P4/05 (SSTC, Belgium).

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